Considerations of energy saving and reduction of pollution, particularly in an urban environment, are leading motor vehicle manufacturers to equip their models with an automatic stopping/restarting system, such as the system known by the term “Stop and Go”.
As stated by the company VALEO EQUIPEMENT ELECTRIQUES MOTEUR in patent application FR2875549, vehicles are enabled to function according to the “Stop and Go” mode by means of a reversible electrical machine, or alternator—starter, which is coupled to the thermal engine, and is supplied by an inverter in “starter” mode.
The use of an alternator—starter in a “Stop and Go” functioning mode consists, in certain conditions, of giving rise to complete stoppage of the thermal engine when the vehicle itself is at a standstill, then restarting the thermal engine, as a result, for example, of an action of the driver which is interpreted as a demand for restarting.
A typical “Stop and Go” situation is that of stopping at a red light. The driver stops the vehicle at the light, the thermal engine is automatically stopped, then, when the light turns green, the engine is restarted by means of the alternator—starter, as the result of detection by the system of the clutch pedal being pressed down by the driver, or any other action which conveys the wish of the driver to restart his vehicle.
It will be appreciated that the function of automatic restarting carried out by an alternator—starter system is a function which must be as transparent as possible for the driver of the vehicle.
In alternator—starters consisting of a polyphase rotary electrical machine with an inductor, the phase currents and the excitation current are generally supplied simultaneously by power circuits at the moment of restarting.
In American patent U.S. Pat. No. 6,335,609, it is found that in these circumstances the engine torque can be produced only with a perceptible delay.
This delay is due to the establishment of a magnetic flux in the rotor, and it is proposed to carry out pre-fluxing of the inductor before the establishment of the phase currents, such as to reduce the time which is necessary for the thermal engine to reach a predetermined speed of rotation.
However, the method is implemented by controlling the excitation current for a fixed duration, and does not appear to be suitable for alternator—starters which are supplied by an on-board network with variable voltage, of the “14+X” type, in so-called “micro-hybrid” systems.
A need therefore exists for a method and a device which make it possible to maintain within limits which are acceptable for the driver the starting time in the case of an architecture of an automatic stopping/restarting system of a micro-hybrid type, in which the voltage of the on-board electrical network depends on the state of charge of the ultra-capacitor.